Structural concrete, though capable of carrying very high compressive loads, is generally weak in carrying tensile loads on its own. Reinforced concrete ameliorates this deficiency by including an internal structure formed from materials capable of withstanding tensile forces within an otherwise solid concrete structure. Metal bars or cables are often used due to their high tensile strength and relative ease of manufacture.
In order to further improve the tensile capacities of reinforced concrete structures, the reinforcement structure may be pre- or post- tensioned. Added structural tension maintains a compression loading on the concrete member, even when tensile stress would otherwise occur, such as in beam-loading.
In post-tensioned concrete, the reinforcing structure is tensioned after the concrete has set. In one form of post-tensioned concrete, a series of conduits formed from duct segments are placed within the concrete form, each conduit positioned parallel to the desired tensile pre-loading. The conduits are threaded with the one or more tensile members, such as metal cables. After the concrete has set, the metal cables may then be placed under tension, and anchored to either end of the conduit, thus placing the concrete member under tensile loading.
Where a concrete member is to be poured adjacent to an existing concrete member, such as, for example, in a balanced cantilever construction bridge, the duct segments in the existing concrete member are traditionally be coupled to ducts placed in the form of the new concrete member.